Things will look far better when Mata and Witsel have signed. If it must happen, then I hope Stones will leave in the next couple of weeks too and not have it drag out.
Still a lot of work for Koeman to do between now and August - a first-choice keeper, Stones replacement, and striker to support Lukaku are still needed. Including Stekelenberg that would make 6 new signings and we could do with cover at RB as well.
I'm happy with the immiment Mata signing at the price, remain to be convinced about Witsel. I'm not sure how 'physical' he is in his approach. One thing that has struck me over recent years is that we have no real "athletes" in the centre - of the mould of say Pogba / Matuidi - tall, strong, pacey players. We have accumulated a lot players who don't seem as imposing physically over the years - like Osman, Besic, McCarthy, Pienaar etc. We have pace with Deulofeu, Mirallas, & Lukaku but I would welcome more pace and aggression in the team particularly if we are going to adopt a pressing game. I would have prioritised Schneiderlin over Witsel simply because he is a known quantity in the PL. Barkley with a proper pre-season and fitness regime behind him could lose 10lbs and become fitter and leaner in that respect. He looked like someone playing rugby league last season.
Other that that, I hope if we make another 3-4 signings that Koeman takes the overall age profile of the squad into account. We have more leeway signing another goalkeeper but I don't think it's wise to accumulate 5-6 new players all in their late twenties or beyond. If we are going to have to continue to sacrifice a big-name player each season for the next few years its something that has to be considered, and it's not good for the squad generally anyway.